Hydrotalcites acetone condensation

Gas-Phase Acetone Condensation over Hydrotalcite-like Catalysts 55... 

A number of solid base catalysts have been reported in the literature to be active for acetone condensation. These include alkali oxides (NajO, K O, CsjO) (7), alkaline earth oxides (MgO, CaO, BaO) (1,4-6), transition metal oxides (7) and phosphates (8-11), ion-exchange resins (12), zeolites (13) and clay minerals and hydrotalcites (HTC) (14,15). A suitable catalyst for the acetone-to-MIBK reaction must have several properties the condensation of acetone to DAA is catalyzed by either basic or acidic sites, the dehydration of DAA to MO is acid-catalyzed, and the selective hydrogenation of MO to MIBK requires appropriate metal sites (7,8). 

An example of commercial interest is the synthesis of citronitrile (Scheme 17), a compound with a citrus-like odor, which is used in the cosmetics and fragrance industries. The first step in the synthesis of citronitrile is the Knoevenagel condensation of benzyl acetone and ethyl cyanoacetate. This condensation has been carried out with MgO and Al-Mg calcined hydrotalcites as catalysts (148). Similar results were obtained with the two solid catalysts, with yields of 75% of the Knoevenagel adduct. 

Figure 2 shows illustrated mechanism for acetone self-condensation over calcined hydrotalcites, where the enolate ion is formed in a first step followed by two possible kinetic pathways 1) In the first case the subtracted proton is attracted by the basic sites and transferred to the oxygen of the enolate ion to form an enol in equilibrium. 2) In the second case the enolate ion reacts with an acetone molecule in the carbonyl group, to produce the aldol (diacetone alcohol). Finally, the p carbon is deprotonated to form a ternary carbon and then loses an OH group to obtain the final products. 

Table 2. Results of self-condensation of acetone over various calcined hydrotalcite-like compounds.

Roelofs, J. C. A. A., van Dillen, A. J. and de Jong, K. P. Base-catalyzed condensation of citral and acetone at low temperature using modified hydrotalcite catalysts, Catal. Today, 2000, 60, 297-303. 

Aldol and related condensation reactions such as Knoevenagel and Claisen-Schmidt condensations are also widely used in the fine chemicals and specialty chemicals, e.g. flavors and fragrances, industries. Activated hydrotalcites have been employed as solid bases in many of these syntheses. Pertinent examples include the aldol condensation of acetone and citral [107, 108], the first step in the synthesis of ionones, and the Claisen-Schmidt condensation of substituted 2-hydroxyacetophenones with substituted benzaldehydes [109], the synthetic... 

Interesting recent developments are the use of hydrotalcite supported on carbon nanofibers [119], to facilitate recovery of the catalyst by filtration, and the use of synthetic hydroxyapatite, Ca10(PO4)6(OH)2 as a solid base catalyst in a variety of reactions including Michael additions [120]. The supported hydrotalcite exhibited higher activities and selectivities than the conventional unsupported material in the aldol condensation of citral with acetone [119]. 

Cesium-exchanged zeolite X was used as a solid base catalyst in the Knoevenagel condensation of benzaldehyde or benzyl acetone with ethyl cyanoacetate [121]. The latter reaction is a key step in the synthesis of the fragrance molecule, citronitrile (see Fig. 2.37). However, reactivities were substantially lower than those observed with the more strongly basic hydrotalcite (see earlier). Similarly, Na-Y and Na-Beta catalyzed a variety of Michael additions [122] and K-Y and Cs-X were effective catalysts for the methylation of aniline and phenylaceto-nitrile with dimethyl carbonate or methanol, respectively (Fig. 2.37) [123]. These procedures constitute interesting green alternatives to classical alkylations using methyl halides or dimethyl sulfate in the presence of stoichiometric quantities of conventional bases such as caustic soda. 

It has previously been reported that hydrotalcite catalyzes the aldol condensation of acetone (25). Polyoxometalates are known to dehydrate alcohols due to their acidic nature (IS ). In order to compare the relative basicity of polyoxometalate-pillared hydrotalcites to that of hydrotalcite itself, a variety of hydrotalcites were screened for 2-propanol conversion (Table II). This reaction is known to give propylene when the catalyst contains acidic sites (such as alumina) and acetone when the catalyst contains basic sites (such as magnesium oxide). 

The aldol condensation of formaldehyde and acetone to methylvinylketone has been investigated by Suzuki and Ono [20] on a series of solids MgAl hydrotalcite was found to be the best catalyst, leading to 21 % conversion of acetone at 673 K, with 96 % selectivity for acetone and 64 % for formaldehyde. The carbonate form was the most suitable. Exchange by sulfates or chromates resulted in poor activity. 

These results, which are highly analogous with those from the homogeneous base-catalyzed condensation reaction, imply that the condensation of benzaldehyde and acetone is base-catalyzed on hydrotalcites. 

The specific properties of hydrated hydrotalcites appear not only in the aldoli-zation of acetone, but in many other aldolization reactions. For example, in the aldol condensation of benzaldehyde with acetone the hydrated form catalyzes the reaction at 273 K, yielding aldol as the main product instead of benzalacetone, obtained on the calcined sample. Competitive adsorption kinetics are still observed, with a much greater adsorption coefficient for benzaldehyde. As suggested earlier from Hammett relationships, this reaction can be generalized with success to many substituted benzaldehydes [32], although the reaction could be performed selectively at 273 K with benzaldehyde only, and substituted benzaldehydes required a reaction temperature of 333 K. Because of this high temperature the reaction usually gives a, unsaturated ketones isolated yields are > 95 %. 

MgAl hydrotalcite impregnated with potassium t-butylate has recently been used for aldolization reactions [35]. The results are summarized in Table 4 for the condensation of acetone on benzaldehydes of general formula R2CHO. 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

The condensation/hydrogenation of acetone to meOiyl isobutyl ketone (MIBK) was studied on a series of hydrotalcite-supported noble metal catalysts in a liquid-phase batch microreactor at 118 C and 400 psig. The O.lwt.% Pd/HTC gave the highest acetone conversion (38%) and selectivity to MIBK (82%). The HTC catalyzes the condensation of acetone to diacetone alcohol (DAA) and its subsequent dehydration to mesityl oxide (MO), whereas the noble metal selectively hydrogenates MO to MIBK. A multifunctional catalyst is required for this acetone-to-MIBK reaction. 

Hydrated hydrotalcite is active for cross-aldol condensations of acetone with various substituted benzaldehyde (22). The aldol was the major product at room temperature but the yield was low. At 333 K, yields were improved to give selectively the corresponding dehydration products of aldol condensation (22). 

Hydrotalcite, an anionic clay mineral with the formula Mg6Al2(0H)i6C034H20, shows a high activity for cross aldol condensation of formaldehyde with acetone to form... 

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